Author Topic: Carburretor Thoughts  (Read 20915 times)

Offline Vixen

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Re: Carburretor Thoughts
« Reply #60 on: March 26, 2020, 07:25:33 PM »
Hi Roger,

This is getting exciting. Can we fast forward?

Mike
It is the journey that matters, not the destination

Sometimes, it can be a long and winding road

Offline Roger B

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Re: Carburretor Thoughts
« Reply #61 on: March 28, 2020, 05:06:41 PM »
Unfortunately you may have to wait a while with my rather slow rate of progress  :(  I still have some new injectors to build for the diesel  ::)

I made a selection of brass weights to go on top of the piston guide rod to try and get an idea of the additional load requires. The biggest, 13mm diameter calculated weight 6g, was going in the right direction. I borrowed a chunkier power supply from work (spare focus magnet power supply for a 3 million volt electron accelerator) so I could try higher rpm. I got up to 5000rpm where the 6g weight was not enough, the motor was also getting rather hot after 10s of seconds running at 25V 17A, over 400w.

<a href="https://www.youtube.com/watch?v=GoPOQdOmHI8" target="_blank">http://www.youtube.com/watch?v=GoPOQdOmHI8</a>

<a href="https://www.youtube.com/watch?v=8c9za4AgkR0" target="_blank">http://www.youtube.com/watch?v=8c9za4AgkR0</a>

I cannot fit a weight that size on top of the piston, I would have to remake the piston in brass. I noted that Amsbury made his piston from mild steel  :thinking: The other option is a spring. I couldn't find anything standard but Lee springs calculator came up with a possible design. Maybe I will have to look at winding my own. Once again the 0.355 wire in the Lee Spring's design is close to the 0.4mm wire in the Amsbury design.
Best regards

Roger

Offline Admiral_dk

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Re: Carburretor Thoughts
« Reply #62 on: March 28, 2020, 07:13:30 PM »
Great result so far on the first tests and thanks for the videos :ThumbsUp:

I know that you're mostly testing - but a spring works in any direction - so I would prefer that if I was you ....
You never know if you suddenly want to have a future vehicle "bumping down the track" so to speak.

 :cheers:    :popcorn:

Offline Roger B

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Re: Carburretor Thoughts
« Reply #63 on: March 29, 2020, 04:45:56 PM »
Thank you Per  :ThumbsUp:

I searched further through the Lee Springs list and found a better option with a rate of 0.02N per mm. The 6g additional weight delivers a force of 0.06N so a compression of 3mm will give that and there will be a rising rate to 0.18N when fully open.

I also think that the bridge under the piston on the full size carb is important for the idling/low speed performance as it defines the venture better than the bottom of the bore. The only sensible way I could see to make one was to bore right through and then make a suitable shaped plug and fix it in place. Boring through is also much easier than a blind bore as will be keeping the bore for the jet concentric.

As I was going to bore through I also decided to turn a short spigot to centre the cover. Amsbury's idea of centring by hand each time is a pain. There was just enough material to make a 0.5mm high spigot without breaking through the bores from the input end. Measuring was interesting but possible. I had a couple of problems facing back out, hence the two dig in grooves but I think I need to redesign the whole top of the body. The cover was a good snug fit so I drilled out the bulk of the material for the bridge plug with a 5.5mm drill and then bored in 0.05mm steps until the tool just touched the existing bore.

It now all assembles easily without any jamming  :)
Best regards

Roger

Offline Vixen

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Re: Carburretor Thoughts
« Reply #64 on: March 29, 2020, 05:23:26 PM »
Hi Roger,

I too believe the bridge under the piston to be an important detail for the low end performance. The proposed insert plug looks like the most sensible way to implement it. It's attention to these fine details that will help you achieve your objective

Have you had any more thoughts about the virtues of an adjustable needle in the piston verse the adjustable jet in the bridge. I believe the adjustable needle has a lot in its favour

Stay in, Stay safe

Mike
It is the journey that matters, not the destination

Sometimes, it can be a long and winding road

Offline Roger B

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Re: Carburretor Thoughts
« Reply #65 on: March 30, 2020, 07:58:05 PM »
Thank you Mike  :ThumbsUp:

I'm not sure how to install an adjustable needle in the piston, the guide rod is 3mm diameter so I would be working with M2 or M1.6 threads and I can't see how to lock that against vibration  :headscratch:

I have started on the bridge plug. Machining the two ramps at 25° required some thought but I came up with a solution of clamping my small Proxxon RT in a vice at the required angle and then turning 180° for the other side.
Best regards

Roger

Offline Vixen

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Re: Carburretor Thoughts
« Reply #66 on: March 30, 2020, 08:31:17 PM »
Thank you Mike  :ThumbsUp:

I'm not sure how to install an adjustable needle in the piston, the guide rod is 3mm diameter so I would be working with M2 or M1.6 threads and I can't see how to lock that against vibration  :headscratch:


Hi Roger, A few turns of cotton wrapped around a M1.6 thread would be sufficient to seal and provide anti vibration. If you were to have a movable needle as well as a movable jet, you could see which one gives the better mixture control. I always felt the mixture enrichment by dropping the jet on the SU was far to crude. It really only provided lots of extra petrol during a cold start, rather than an enriched mixture. Screwing a needle in and out must give better control.

Stay in, stay safe

Mike

PS that bridge looks just right.
« Last Edit: March 30, 2020, 09:29:43 PM by Vixen »
It is the journey that matters, not the destination

Sometimes, it can be a long and winding road

Offline Rick Doane

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Re: Carburretor Thoughts
« Reply #67 on: March 31, 2020, 01:21:13 PM »
Hello Roger:

Attached are the adjustable remote needle valve assemblies I use in rc boats.  They come in two sizes, course and fine needles.  I'm not sure if you could use anything like these but they work very well for adjusting lean or rich settings for my application.

Regards....Rick

Offline Roger B

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Re: Carburretor Thoughts
« Reply #68 on: March 31, 2020, 07:43:11 PM »
Thank you Rick, those are interesting. Where do you get them from? I don't think they will help me directly in this carb but they are good small control valves for controlling the flow to a overflow type fuel feed when I get that far.

Mike, I think that if I can adjust the jet height with a fine thread (0.5mm pitch or less) it will be easier that putting a hex key or watch makers screwdriver down the piston guide. The Stromberg CD carb had an interesting mixture enrichment for starting. There was a rod with a flat on it as the manifold side of the bridge which was rotated to lift the piston and increase the fuel orifice as well as restricting the air flow.
Best regards

Roger

Offline Vixen

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Re: Carburretor Thoughts
« Reply #69 on: March 31, 2020, 07:58:59 PM »

Mike,  The Stromberg CD carb had an interesting mixture enrichment for starting. There was a rod with a flat on it as the manifold side of the bridge which was rotated to lift the piston and increase the fuel orifice as well as restricting the air flow.


Roger, That is a clever way to do the enrichment. Does it look feasible in model scale, the lift would only be 1mm or so. May have a problem with air leaks at the side of the rod, where it goes through the main body?
 This way  sounds a better way than the SU way of dropping the jet down inside the bridge, which gives poor atomisation.
Quote

I think that if I can adjust the jet height with a fine thread (0.5mm pitch or less) it will be easier that putting a hex key or watch makers screwdriver down the piston guide.


In my minds eye, I saw the adjustable needle going all the way down the 3mm diameter x 29 mm central rod in the piston. The adjuster knob would be outside at the top.

Stay in, stay safe

Mike
« Last Edit: March 31, 2020, 08:23:23 PM by Vixen »
It is the journey that matters, not the destination

Sometimes, it can be a long and winding road

Offline Roger B

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Re: Carburretor Thoughts
« Reply #70 on: April 01, 2020, 05:22:39 PM »
Thank you Mike, I now understand your ideas about the adjustable needle  :ThumbsUp: I shall give it some thought  :thinking:

Here is some more information on SU and Stromberg CD carbs for those who may be interested:

http://sucarb.co.uk/technical-su-carburetters

http://zenithcarb.co.uk/zencdcdsseries/?___store=zenith
Best regards

Roger

Offline Roger B

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Re: Carburretor Thoughts
« Reply #71 on: April 04, 2020, 05:36:32 PM »
I decided to use some medium strength Loctite and a grub screw to retain/seal the bridge plug in case I need to change the profile.

Following the original route while I think about the adjustable needle I started on the jet block. The was made from a  6mm slice of 12mm square brass. On the first attempt the while drilling for the jet the drill dug into the hole for the petrol feed union and gave an oval hole. For the next attempt I finished both holes with a 3mm 'throwaway' milling cutter.
Best regards

Roger

Offline Roger B

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Re: Carburretor Thoughts
« Reply #72 on: April 04, 2020, 07:52:59 PM »
A quick check that the stop screw clears the jet seal block and then on with the pivot support and jet adjustment lever.
Best regards

Roger

Offline Roger B

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Re: Carburretor Thoughts
« Reply #73 on: April 05, 2020, 05:42:36 PM »
The next step was to try and make a needle. I had scaled to 0.5mm diameter from the Amsbury design but felt it might be a step too far. The tapered part needed to be around 8mm long, 5mm for the bore and then 3mm to allow some jet drop. Starting with some 1.5mm diameter silver steel I tried to turn it down to 0.5mm. Using a new aluminium insert in the Proxxon lathe at 3000 rpm the first 0.2mm cut was ok, the next at 0.1mm failed  :(

I then set the slide over at 1° (I don't know if this is a realistic angle or not  :headscratch: ) and tried again. I took a first cut at 0.25mm and then dropped to 0.05mm. The workpiece was visibly deflecting so I reduced the cuts to 0.025mm (It did cut and produce some hair like curly swarf). I got down to 0.5mm but the deflections meant that the needle was almost parallel  :(

Maybe 0.5mm is not practical so I tried again to produce a 0.8mm needle. This was cut using similar steps to before to reach a starting diameter of 0.8mm. The tip was 0.6mm which calculates as an included angle of 1.4° rather than 2°, probably due to deflection.

That was enough difficult bits so I decided to make the fuel connection nipple for the jet block. This was silver soldered in place as I expect I will have to change the jet more than once so that will be soft soldered to allow easy removal.
« Last Edit: April 05, 2020, 05:49:14 PM by Roger B »
Best regards

Roger

Offline Admiral_dk

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Re: Carburretor Thoughts
« Reply #74 on: April 05, 2020, 08:58:28 PM »
The final result looks good - but I'm sure it was hard work to get down to so small dimensions  :ThumbsUp:

I can't help wondering if it almost requires a tree sided cutter ala some thread dies to avoid deflection  :noidea:

 

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